Constant composition recycle of nylon 6 polymerization wash water

ABSTRACT

A process for preparing polyamides from amide monomers in a reaction vessel where a portion of the monomers remain unreacted or form oligomers as an extractable fraction includes extracting from the polyamide unreacted amide monomers and oligomers to yield an extracted proportion by weight based on total reaction charge; reducing the oligomer concentration of the extracted proportion relative to the total extract concentration; and returning the extract to the reaction vessel as substantially the same proportion as extracted by weight based on the total reaction charge, so that, after a single extraction, throughout the process the extractable fraction remains below its equilibrium concentration.

RELATED APPLICATIONS

This application is a continuation-in-part of copending U.S. patentapplication Ser. No. 07/531,976 filed Jun. 1, 1990, now U.S. Pat. No.5,077,381.

BACKGROUND OF THE INVENTION

This invention relates generally to amide polymerizations. Morespecifically, this invention relates to the recovery and recycle ofunreacted amide monomers and oligomers in ε-caprolactam polymerizations.

In the polymerization of ε-caprolactam, about 10% of the caprolactammonomer remains unreacted. In addition, about 2.5% total cyclicoligomers form. The art has recognized the need to recover and recyclethese valuable raw materials. Recovered materials are useful in laterpolymerization reactions or for other purposes. Further, if left in thepolycaprolactam product, residual monomer and oligomers causeundesirable effects in further processing of the polymer product.

A typical recovery process involves hot water extraction of residualmonomer and oligomers and recovery of the extractables by evaporatingthe water. In many cases, a first distillation step isolates thecaprolactam monomer while the oligomer is depolymerized and distilled inlater steps.

U.S. Pat. No. 4,053,457 to Cordes et al. describes caprolactam recovery.Cordes et al. discloses a process for extracting ε-caprolactam and otherpolyamide-forming starting materials. The extract containing solvent,monomer and oligomers is concentrated in the absence of atmosphericoxygen, the surfaces which come into contact with the extract being madeof materials which are inert under the conditions of the concentrationprocess, and the concentrate, without further purification orseparation, is polymerized by itself or together with otherpolyamide-forming compounds.

U.S. Pat. No. 4,540,774 to Gerking et al. discloses a process forcontinuously demonomerizing and postpolymerizing polycaprolactam meltsin a reactor designed for carrying out the process. Demonomerization isby vacuum. Condensed monomers and oligomers may be recycled, in whole orpart, to the process origin.

U.S. Pat. No. 4,816,557 to Pipper et al. discloses a process forremoving caprolactam monomer and oligomers thereof from nylon granuleswherein the nylon granules are heated and superheated steam is passedtherethrough in a treatment zone. The steam containing caprolactam andits oligomers is withdrawn from the top of the treatment zone. Thissteam then passes through a column to yield an aqueous solution ofcaprolactam and oligomers thereof and caprolactam free steam. Accordingto the disclosure, the steam may be recycled.

As noted, it is known that recycling monomers left unreacted inpolymerization reactions is advantageous. For instance, recyclingmonomer to fresh polymerizations preserves raw materials. In largevolume operations, the conservation of raw materials can result inconsiderable cost savings. Other factors may merit consideration, too.As an example, recycling raw materials avoids the problem of wastedisposal, a growing environmental concern.

For example, U.S. Pat. No. 4,537,949 to Schmidt et al. describes acontinuous process for preparing certain nylons wherein prepolymer andvapors are continuously separated, the vapors are rectified and theentrained diamines are recycled. Although the Schmidt et al. disclosuredoes not specifically address ε-caprolactam monomers, the teachingsrecited highlight the importance given to recycling in general.

U.S. Pat. No. 4,429,107 to Strehler et al. discloses a process forcontinuously preparing polycaprolactam in which ε-caprolactam ispartially polymerized, with the addition of a water-containing agent andacetic acid or propionic acid as a chain regulator, at a nylon-formingtemperature. A gaseous mixture of caprolactam, water and acetic orpropionic acid obtained at the top of the reactor is fed to the middleof a column. Water is removed at the top of the column and the bottom ofthe column is maintained at 125° C. to 145° C. The mixture obtained atthe bottom of the column contains caprolactam, acetic or propionic acidand a small amount of water and is recycled to the top of the reactor.

U.S. Pat. No. 4,327,208 to Lehr et al. discloses a process for producingpolyamide-6 or corresponding copolyamides by hydrolytic polymerizationwherein the low molecular weight secondary reaction products and theunreacted ε-caprolactam are separated from the polyamide melt anddirectly condensed on an ε-caprolactam melt intended for polymerization.Reduced pressure and elevated temperature result in the separation ofthe monomers and oligomers from the polyamide melt. No intermediatetreatment of the separated material takes place between separation andcondensation. Lehr et al. also discusses the significance of cyclicdimer formation in the polymerization of ε-caprolactam.

Cyclic dimer forms during the initial ring opening of ε-caprolactam inthe polymerization. The cyclic dimer is relatively stable and has a highmelting point and low solubility. Moreover, cyclic dimer is notadequately processable at conditions and parameters suitable forpolycaprolactam. One of the problems associated with cyclic dimer is itsdeposition on processing machinery which causes downtime in the mill forequipment cleaning. Much attention has focused on removing cyclic dimerfrom polyamides, particularly polycaprolactam.

Exemplary is U.S. Pat. No. 4,310,659 to Yates et al. describing apolymerization of ε-caprolactam which uses a two stage hydrolysisprocess. The first stage operates at a first temperature and a firstpressure. Before equilibrium conditions are reached a second stageoperates at a second temperature and pressure while water iscontinuously removed. As a result, water (containing hydrophilicextractables) is removed both during hydrolyzation and during thesubsequent polycondensation so that the cyclic dimer content of theshaped polymer article is below 0.2 percent by weight.

U.S. Pat. No. 4,436,897 to Strehler et al. discloses a process forpreparing polycaprolactam by polymerizing ε-caprolactam, and an aqueousextract containing ε-caprolactam and caprolactam oligomers. The extracthas been obtained by extracting polycaprolactam with water sufficient toachieve an extract containing from 0.1 to 5.0% by weight of oligomers ofcaprolactam, based on the monomeric caprolactam in the aqueous extract.This extract is then used in later polymerizations. As disclosed, afterthe above extraction, a second extraction yields residual extract with ahigher dimer concentration.

What remains needed is a method for continuously recycling wash waterconcentrate from amide polymerizations to the reaction vessel withoutdilution with virgin monomer prior to concentrating while establishing acyclic dimer concentration, relative to the reactants, well below itssolubility equilibrium concentration. In addition, there remains a needfor a process whereby extractables from amide polymerizations can berecovered and returned as a polymerization starting material which, evenafter a single extraction step, keeps the extractables below theirequilibrium concentration throughout the continuous polymerization.

SUMMARY OF THE INVENTION

Accordingly, the present invention is an improvement in a process forpreparing polyamides from amide monomers in a reaction vessel where aportion of the monomers remain unreacted or form oligomers as anextractable fraction. The invention includes the steps of extractingfrom the polyamide unreacted amide monomers and oligomers to yield anextracted proportion by weight based on total reaction charge; reducingthe oligomer concentration of the extracted proportion relative to thetotal extract concentration; and returning the extract to the reactionvessel as substantially the same proportion as extracted by weight basedon the total reaction charge, so that, after a single extraction,throughout the process the extractable fraction remains below itsequilibrium concentration.

An advantage of the present invention is an improved polyamidepolymerization process.

Related advantages and objects of the invention will be readilyascertainable by the ordinarily skilled artisan after a reading of thefollowing description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To promote an understanding of the principles of the present invention,descriptions of specific embodiments of the invention follow andspecific language describes the same. It will nevertheless be understoodthat no limitation of the scope of the invention is thereby intended,and that such alterations and further modifications and such furtherapplications of the principles of the invention as discussed arecontemplated as would normally occur to one ordinarily skilled in theart to which the invention relates.

In the discussion which follows, the term "extractants" is used to referto the non-solvent portion of the extract from polycaprolactamproduction. The term is used without regard to the stage or priortreatment of the extract. Percentages are weight percentages unlessotherwise designated.

In the following description, the cyclic dimer of ε-caprolactamconcentration has been measured by gas chromatography. One suitablemethod is described more thoroughly below. Of course, other analyticalmethods and instruments might be used. The present invention applies topolycaprolactam production according to any acceptable method known tothose ordinarily skilled in the relevant art. In addition, the presentinvention is useful in copolymerizations of ε-caprolactam with othermonomers. Further, the present invention is advantageously used in anypolymerization producing troublesome forms and wherein it is desirableto recycle unreacted starting materials.

In practice, the present invention generally involves the step ofextracting unreacted caprolactam monomer and oligomers from polymerizedε-caprolactam. The resulting extract is typically then concentrated.This concentrated extract is commonly known as concentrated wash water.Heat and pressure are then applied to the concentrated wash water for aperiod of time and the cyclic dimer concentration is accordinglyreduced. The concentrated wash water having reduced cyclic dimerconcentration is then returned to the reaction vessel without furtherdilution and in an amount such that the extractants are returned to thereaction vessel as about the same fraction as removed. Stated anotherway, if about 10% of the original polymeric product was extracted, theextractants are returned to the reaction vessel as about 10% of the newcharge of reactants. Surprisingly, when extractables are removed andrecycled in this way, only a single extraction step is required tomaintain the extractables below their equilibrium concentrationthroughout the polymerization process.

The extraction of polymerized caprolactam results in an extract havingapproximately 10% total organics by total weight of the extract,including the weight of the extracting solvent. These total organicstypically include about 6 to 8 percent monomer and about 2.5 percentcyclic oligomers of which about 0.5 percent is cyclic dimer. Theextractables represent approximately 10% of the monomer initiallycharged to the reaction vessel.

Water is commonly the extracting solvent of choice. Acceptablesubstitutes for water will be readily apparent to those of ordinaryskill. For example, steam may be used and later condensed. When water isused, an acceptable rate of water usage is, for example, about 0.8 to2.0 liters per kilogram.

The extract is then typically concentrated by evaporating the solvent torecover the extractables. After concentration, the extract orconcentrated wash water contains approximately 80% total organics. Ofthis approximately 5%, more or less, represents cyclic dimer.

According to the present invention, the concentrate is then hydrolyzedto reduce the cyclic dimer concentration. Presently this is accomplishedby charging the concentrated wash water to a pressure reactor where itis heated under pressure for a period. After treatment in the pressurereactor, the cyclic dimer concentration can be reduced from about 5% toapproximately 1.3%, more or less.

Hydrolysis of cyclic dimer preferably takes place under autogenouspressure of an aqueous solution containing about 80% organics. Thepreferable temperature for the hydrolyzation is between about 220° C.and about 290° C. and is most preferably about 265° C. The hydrolyzationmay be allowed to proceed for any period of time which is suitable toadequately reduce the cyclic dimer concentration. This time is, forexample, preferably from about 2 to about 6 hours, and most preferablyis in about the 4-hour range.

Once the hydrolysis of the dimer is complete the resulting wash waterconcentrate with reduced cyclic dimer concentration may be returned tothe reactor as starting material for the next polymerization. It iscontemplated that extractables will be removed continuously from theproduct polymer, hydrolyzed and returned to the reaction vessel as partof the continuous raw material feed. According to the invention, thecontinuous removal of the extractables may be a single water (or othersolvent) extraction step.

According to the invention, the hydrolyzed material is preferablyreturned to the reactor in about the same proportion as it was removedfrom the polymer in order to maintain a constant composition of thecyclic dimer which is well below its equilibrium concentration. Forexample, if the total organic content of the unconcentrated extract isabout 10% then the concentrated extract which has been hydrolyzed shouldbe returned to the reactor in an amount which is approximately 10% ofthe total raw material charge. About ninety percent (90%) of thecaprolactam charge is, for example, virgin caprolactam and other freshstarting materials. The cyclic dimer concentration is accordinglyapproximately 0.13% of the total material charge.

Because the hydrolyzed extracted cyclic dimer is recycled at a ratesubstantially the same as the extraction rate, the concentration ofcyclic dimer will not reach its equilibrium concentration ofapproximately 1.3% even when the dimer is extracted in a single step.After the second introduction of recycled material, an approximatelysteady concentration of dimer results. By way of illustration, if eachpolymerization cycle forms about 0.5% new cyclic dimer and approximately0.13% cyclic dimer is returned in the recycled extract (according to thedescription above), then the concentration of cyclic dimer in thepolymeric product will not exceed about 0.63%, a concentration wellbelow the equilibrium concentration of cyclic dimer. Of course, if theprocess of the present invention is used in other than the production ofhomopolymeric polycaprolactam, these concentration values will varyrespectively. Because of this steady concentration which is well belowthe equilibrium concentration, the extraction may be in a single step.

The invention will now be further described by reference to thefollowing more detailed examples. The examples are set forth by way ofillustration and are not intended to limit the scope of the invention.

Determination of Cyclic Dimer in Nylon by Gas Chromatography

A 10-gram sample of nylon is refluxed in 100 ml of methanol for 3 hours.When the mixture is cool, 5 ml of methanol is added through the top ofthe condenser. The sample is centrifuged and the methanol extract isthen injected into a gas chromatograph for analysis of cyclic dimer.Quantitation is by external calibration. Calibration standardscontaining 0.03%, 0.5% and 0.08% cyclic dimer are prepared by adding to10 ml of methanol 0.003, 0.005 and 0.008 grams of cyclic dimer,respectively.

Sample Preparation:

10 grams of sample are weighed into a 250 ml Erlenmeyer flask. Chipsshould be ground before refluxing and yarn should be cut into smallpieces. One hundred (100) ml of methanol is added to each sample flask.The mixture is refluxed for 3 hours and when cool, 5 ml of methanol isadded to the top of the condenser. The mixture is centrifuged.

Calibration Curve:

Using a 10 uL syringe with a chaney adaptor, 1 uL of each standard isinjected into a Varian Model 3700 gas chromatograph equipped with aflame ionization detector and a 6 ft glass 1/4 inch OD by 2 mm IDcolumn, packed with 3% OV 17 on Gas Chrom Q, 80/100 mesh. The carriergas is helium at between 30 to 40 mL/minute. The column temperature isbetween 215° C. to 220° C. The injector temperature is 280° C. and theflame ionization detector is maintained at 280° C. Each standard isinjected at least twice. The cyclic dimer peak areas are calculated andaveraged. A standard calibration curve results by plotting the cyclicdimer peak area versus concentration in weight/volume percent.

Sample Concentration:

1.0 uL of each sample is injected at least twice and the cyclic dimerpeak area recorded. Peak areas are averaged.

Calculations:

The concentration of cyclic dimer in the sample is calculated using thefollowing equation: ##EQU1## C=concentration of cyclic dimer in thesample (wt. %) A=concentration of cyclic dimer in sample solution readfrom calibration curve (wt/vol %)

V=Total volume of methanol added, mL

W=Weight of sample, g

EXAMPLE 1 Extraction of Nylon 6 and Hydrolyzation of the Extract

Polycaprolactam pellets containing about 12.5 wt % extractable material,monomer and oligomers, are extracted once with water at a temperature ofabout 97° C. at a rate of 1.2 liter of water per kilogram of pellets.Caprolactam and cyclic dimer concentrations are determined by gaschromatography after the method described above. The concentrations ofhigher cyclic and linear oligomers are determined by difference. Totalorganics are determined by methanol extraction and subsequent analysisby the Kjeldahl method. After extraction the pellets contain about 1.0%extractables and the water contains about 7.9% organics of which about7.2% is caprolactam and 0.4% is cyclic dimer. The remainder of theorganics are higher cyclic and linear oligomers of polycaprolactam.

The extract water is concentrated by evaporation to give concentratedwash water containing about 80% organics of which 66.8% is caprolactam,3.5% cyclic dimer and the remainder is higher cyclic and linearoligomers.

These concentrated extract waters are then heated in a sealed tube in aPaar autoclave at 265° C. under autogenous pressure for 4 hours. Theautoclave is cooled to room temperature and the contents of the tube aredetermined to contain 8.6% caprolactam and 1.3% cyclic dimer. Theremainder of the organics are in the form of aminocaproic acid and othernylon oligomers.

EXAMPLE 2 Polymerization of Virgin ε-Caprolactam

30 grams of ε-caprolactam is charged into a polymerization reactor alongwith 1 gram of water and 2 drops phosphoric acid. The polymerizationinitiates and continues for 16 hours at 260° C. A nitrogen blanket ismaintained during the polymerization. After this time the reactor ispurged with nitrogen. The polymerized material is extruded, cooled,ground into chips and analyzed. Results are presented in Table 1.

EXAMPLE 3 Polymerization of ε-Caprolactam Recovered From Wash Water(Wash Water Unhydrolyzed)

26.7 grams of ε-caprolactam is charged to a polymerization reactor alongwith 4.1 grams (13% of charge) of unhydrolyzed wash water concentrateand 2 drops phosphoric acid. The wash water concentrate is prepared byextraction and in concentration substantially according to Example 1,but without hydrolysis. The mixture is polymerized according to themethod described in Example 2. Results of an analysis appear in Table 1.

EXAMPLE 4 Polymerization of ε-Caprolactam Recovered From Wash Water(Wash Water Hydrolyzed)

26.7 grams of ε-caprolactam is charged to a polymerization reactor alongwith 4.1 grams (13% of charge) of hydrolyzed wash water concentrate and2 drops phosphoric acid. The wash water is prepared by extraction,concentration and hydrolyzation substantially according to the proceduredescribed in Example 1. The polymerization is according to Example 2.Results of an analysis appear in Table 1.

EXAMPLE 5 Polymerization of ε-Caprolactam Recovered From Wash Water(Polymerization Mixture Hydrolyzed)

26.7 grams of ε-caprolactam is charged to a polymerization reactor alongwith 4.1 grams (13% of charge) of unhydrolyzed wash water concentrateand 2 drops phosphoric acid. The wash water concentrate is prepared byextraction and concentration, but not hydrolysis, according toExample 1. The entire mixture is autoclaved at 265° C. for 4 hours. Itis then ground into chips and polymerized according to the procedurestated in Example 2. Results of an analysis appear in Table 1. PG,17

                  TABLE 1                                                         ______________________________________                                        Example % Lactam   % CD     RV*   % Extractable                               ______________________________________                                        2       8.16       .62      3.06  9.90                                        3       8.10       .92      3.08  10.45                                       4       7.32       .63      2.64  10.18                                       5       5.30       .97      2.76  8.85                                        ______________________________________                                         *1 gram polymer per deciliter of 96% sulfuric acid.                      

From this data, it is apparent that, in accordance with the presentinvention, wash water can be recycled to the fresh reactor chargewithout causing the cyclic dimer to reach or exceed its equilibriumvalue.

EXAMPLE 6 Continuous Polymerization of ε-Caprolactam

In a one-stage VK-tube a polymer is produced from 30 grams ofε-caprolactam, 1 gram water and 2 drops phosphoric acid after 16 hoursat about 260° C. The polymer is cooled and made into chips. The chipsare extracted with water to obtain extractable material representingabout 12.5 wt % of the polycaprolactam pellets. Analysis of this extractreveals that it contains about 0.4% cyclic dimer.

The extracted water is concentrated by evaporation to give aconcentrated wash water containing about 80% organics of which 66.8% iscaprolactam, 3.5% cyclic dimer and the remainder is higher cyclic andlinear oligomers. This concentrated extract is heated in a sealed tubein a Paar autoclave at about 265° C. for 4 hours. The autoclave iscooled to room temperature and the contents of the tube are determinedto contain about 8% caprolactam and about 1.3% cyclic dimer.

The resulting hydrolyzed wash water (3 grams) is added to a fresh chargeof caprolactam (27 grams) in an amount representing about 10 wt % of thetotal charge. The mixture is polymerized as described above. The polymerproduced is cooled, cut into chips and analyzed. It contains about 0.53%cyclic dimer.

What is claimed is:
 1. In a process for preparing polycaprolactam fromcaprolactam in a reaction vessel where a portion of the monomers remainunreacted or form oligomers as an extractable fraction, the improvementcomprising:extracting from the polyamide unreacted caprolactam andcaprolactam oligomers to yield an extracted proportion by weight basedon total reaction charge: reducing the oligomer concentration of theextracted proportion relative to the total extract concentration; andreturning the extracted proportion after said reducing to the reactionvessel as substantially the same proportion as extracted by weight basedon the total reaction charge, so that, after a single extraction,throughout the process, the extractable fraction remains belowequilibrium concentration thereof.
 2. The process of claim 1 whereinsaid extracting is accomplished in a single extraction step.
 3. Theprocess of claim 1 further comprising the steps of concentrating theextracted proportion prior to said reducing.
 4. The process of claim 1wherein said extracting is with water.
 5. The process of claim 1 whereinthe extracted proportion contains ε-caprolactam.
 6. The process of claim1 wherein the extracted oligomers contain cyclic ε-caprolactam dimer ina concentration of more than about 1.3% by weight of the extract andsaid reducing reduces the cyclic ε-caprolactam dimer concentration toless than or equal to about 1.3% by weight of the extract.
 7. Theprocess of claim 1 wherein said reducing is by subjecting the extractedproportion to superambient pressure.
 8. The process of claim 1 whereinsaid reducing is by subjecting the extracted proportion to elevatedtemperature.
 9. A process for recycling wash water extract frompolymerization of ε-caprolactam monomers comprising the steps of:(a)extracting organics containing cyclic ε-caprolactam dimer which remainunreacted in the polymerization to produce an extract representing aproportion of the total weight of organic reactants: (b) concentratingthe resulting extract; (c) subjecting the resultant concentrated extractto superambient pressure and temperature; and (d) returning theconcentrated extract after said subjecting to the reactor in aproportion by weight substantially the same as the proportion in whichthe organics were extracted, so that throughout the polymerization thecyclic ε-caprolactam dimer remains below equilibrium concentrationthereof as measured after a single extraction step.
 10. The process ofclaim 9 wherein said extracting is with water.
 11. The process of claim9 wherein said extracting is accomplished in a single extraction step.12. The process of claim 9 wherein the extract contains about 10% byweight of the organic polymerization reactants.
 13. The process of claim12 wherein the proportion in said returning is about 10% by weight ofthe organics charged to the reactor.
 14. A method of maintaining, in apolymerization reactor, recycled ε-caprolactam cyclic dimer belowequilibrium concentration thereof in the polymerization of amidemonomers comprising:(a) extracting unreacted cyclic dimer only once fromthe polymerization reactor; (b) hydrolyzing the extract; and (c)returning the hydrolyzed extract to the polymerization reactor insubstantially the same proportion as removed.
 15. The method of claim 14abd further comprising concentrating the extract prior to saidreturning.
 16. The method of claim 15 wherein said concentrating isprior to said hydrolyzing.